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README.md

1Credit goes to: [grymoire.com](https://www.grymoire.com/Unix/Sh.html#uh-3), [drewdevault.com](https://drewdevault.com/2018/02/05/Introduction-to-POSIX-shell.html), [standard](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html)
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3These are my personal reference notes, I do not claim the examples or definitions below as originally authored by myself
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5## What is POSIX?
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7POSIX (Portable Operating System Interface) is the standard UNIX shell - standard as in formally defined and shipped in a published standard, making shell scripts written for it portable. POSIX, is essentially a formalized version of Bourne.
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9It is found in /bin/sh, although it may be symlinked to bash or you can symlink to dash for performance

posix.txt

1Command substitution
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3Back quotes aren't used to prevent interpretation of special characters it *instead* has a special use - _command substitution_. The string between backquotes is executed by the shell, and the results *replace* the backquoted string:
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5	$ echo 'The current directory is `pwd`'
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7The current directory is `pwd`
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9	$ echo 'The current directory is \`pwd\`'
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11The current directory is \`pwd\`
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13	$ echo "The current directory is `pwd`"
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15The current directory is /home/avsbq
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17	$ echo "The current directory is \`pwd\`"
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19The current directory is `pwd`
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21This feature hail from earlier shells (C and Bourne). This has a major problem - command substitution cannot be nested. A new mechanism was created for command substitution, which replaces the back quotes:
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23`$(command)`
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25Using the above, the previous examples would be:
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27	$ echo 'The current directory is $(pwd)'
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29The current directory is $(pwd)
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31	$ echo "The current directory is $(pwd)"
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33The current directory is /home/avsbq
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35	$ echo "The current directory is \$(pwd)"
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37The current directory is $(pwd)
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39An example command substitution of adding one to the current year.
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41	YEAR="$(date +%Y)"
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43	YEAR="$(expr $YEAR + 1)"
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45	echo "next year is $YEAR"
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47Alternatively we can use nest expressions:
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49	echo "next year is $(expr $(date +%Y) + 1)"